In applications where the input voltage is high, integrated circuit devices may need to pass a “smoke test”. During the test, each input or output (I/O) pin of the integrated circuit device is connected to a high DC voltage, and if the device does not catch fire or smoke, then that particular pin passes the test.
Conventionally, ESD protection structures are used in integrated circuit devices to protect circuitry from being damaged due to electrical static discharge experienced at the I/O pins of the device. A prior ESD protection structure is illustrated in FIG. 1. It comprises a Zener diode D1, a NPN bipolar transistor Q1 and a resistor R1, connected as shown in the figure. The I/O pad and the ground (GND) pad are respectively coupled to the I/O pin and the GND pin.
FIG. 2 is a waveform of the ESD protection structure shown in FIG. 1. The voltage between the I/O pad and GND pad (V) will almost be clamped at a voltage equaling to the breakdown voltage Vbr1 of the Zener diode D1 plus the base to emitter voltage Vbe of transistor Q1. If for a short duration, an ultra-high electrical static voltage, which may be thousands of volts or even higher, is applied between the I/O pin and the GND pin, this clamp voltage is low enough to protect the internal circuitry of the device. However, if the I/O pin is shorted to a DC input voltage (VIN) pin which receives a high voltage during operation, the device will heat up and likely catch fire or smoke due to the high current flowing through the ESD protection structure. Thus, the I/O pin will likely fail the smoke test.
One solution is coupling a metal fuse between the I/O pad and the ESD structure, as shown in FIG. 3. When the I/O pin is shorted to the VIN pin, the fuse will blow to disconnect the ESD structure from the I/O pad. Thus, the device is prevented from over heating and catching on fire or smoking. However, since the fuse is blown, the device is damaged and can not be further used. Furthermore, the metal fuse is unreliable since its width is very narrow and its breakdown current may widely vary.